1. Field of the Invention
The present invention relates to a cathode-ray tube apparatus, and more particularly to a color cathode-ray tube apparatus that is configured to form a fine beam spot on an entire phosphor screen and to stably provide a good image quality with high resolution.
2. Description of the Related Art
In recent years, with an increasing popularity of Hi-Vision broadcast and Internet TV, there is a demand for exact reproduction of an image with higher definition. In order to meet the demand, it is necessary to make pixels finer. In order to display a high-definition image, it is required to form a small-sized beam spot with less distortion over the entire phosphor screen.
In a generally known method for forming a small beam spot on the phosphor screen, a virtual object point size of an electron beam is reduced (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2000-331624). Specifically, of the electrodes that constitute a prefocus lens, the third grid is connected to a resistor for dividing an anode voltage. Thereby, the third grid is supplied with a high voltage. The second grid of the prefocus lens is supplied with a low voltage. Hence, a large potential difference is provided between the second grid and the third grid. In short, a prefocus lens with a strong prefocus function is formed.
Thereby, a great potential permeation occurs into an electron beam passage hole in the second grid G2, and a function for reducing the virtual object point size can be realized. Accordingly, a small beam spot can be formed on the phosphor screen. Moreover, a divergence angle of the electron beam can be reduced by the strong prefocus function. Therefore, the effect of aberration at the time the beam passes through the main lens can be reduced.
In the meantime, in a color cathode-ray tube apparatus that employs an in-line electron gun assembly for generating three electron beams arranged in line in the horizontal direction, the deflection yoke is configured to generate non-uniform deflection magnetic fields. Due to the deflection magnetic fields, a haze of the beam spot appears on the phosphor screen, in particular, on a peripheral region of the screen.
In order to reduce the haze, a method of forming a prefocus lens, which has an astigmatic function with a stronger focusing power in the vertical direction than in the horizontal direction, is generally adopted. In some examples of this method, a horizontally elongated slit is formed at a peripheral region of the electron beam passage hole on the third grid-side part of the second grid, or a vertically elongated slit is formed at a peripheral region of the electron beam passage hole on the second grid-side part of the third grid.
However, if the method of applying a high voltage to the third grid and increasing the permeation of potential into the electron beam passage hole in the second grid is adopted in order to form a small beam spot and if the method of forming the slit at the peripheral region of the electron beam passage hole in the second grid or the third grid to provide astigmatism is adopted in order to reduce the haze of the beam spot, the astigmatic function is intensified in accordance with an increase in the lens power of the prefocus lens.
In other words, the electron beam that passes through the prefocus lens is vertically overfocused and horizontally excessively diverged. As a result, the beam spot on the phosphor screen is distorted and the image quality deteriorates.
To cope with this, the astigmatic function can be designed to decrease by reducing the depth of the slip that is formed in the second grid or the third grid. However, if the depth of the slit in the second grid or the third grid is decreased and the strong prefocus lens is formed, a variation in beam spot shape becomes sensitive to non-uniformity in precision of slit formation or precision in assembly of the electron gun. Consequently, such a problem arises that degradation in image quality tends to easily occur. As a result, it becomes difficult to stably obtain a high image quality.
As has been described above, in the color cathode-ray tube apparatus, in order to display a high-quality image with high definition and high resolution, it is necessary to form a small beam spot with little elliptical distortion over the entire phosphor screen. In addition, in order to realize such a performance, it is necessary to stably manufacture color cathode-ray tube apparatuses with little non-uniformity.
In order to achieve this, the high-voltage side electrode (e.g. third grid) of the prefocus lens is supplied with a high voltage (e.g. a voltage that is higher than the potential of the low-voltage side electrode of the main lens and is lower than the voltage of the high-voltage side electrode of the main lens). Thereby, the lens power is increased, and the permeation voltage, which permeates to the electron beam passage hole in the low-voltage side electrode (e.g. second grid) of the prefocus lens, is increased. Thus, a small beam spot can be formed on the phosphor screen.
However, a variation in lens power per unit dimension of the electrodes of the prefocus lens increases. Consequently, if a slit structure for providing an astigmatic function is added to the electrode of the prefocus lens, the intensity of the prefocus lens action becomes non-uniform, and it becomes impossible to stably form a beam spot with a good shape. In short, the above-described methods cannot provide a sufficiently small beam spot with stable characteristics.